Television pickup tube



Dec. 16, 1952 R. THEILE TELEVISION PICKUP TUBE Filed May 23, 1950Inventor A Horn e y Patented Dec. 16, 1952 UNITED STATES PATENT OFFICETELEVISION PICKUP TUBE British company Application May 23, 1950, SerialNo. 163,658 In Great Britain October 25, 1949 Claims.

The present invention relates to television pick-up tubes of the kindemploying high velocity scanning, such as of the types known asiconoscopes and image iconoscopes. Such tubes suffer from variousdisadvantages, particularly the generation of spurious signals,insufiicient collection by the collector electrode of the tube ofsecondary electrons emitted from the storage surface of the tube, andredistribution effects 'due to secondary electrons emitted from thestorage surface being redistributed over that surface. It is known toimprove the operation of such tubes by irradiating the inside of thetube with a biasing light, and this method is already practiced withtubes of the iconoscope type.

The reason for the improvement is not yet clearly understood in the art,but as a result of practical experience with these tubes and of furthertheoretical investigation, it appears that the improvement is due to thecollection by positively charged parts of the storage surface ofphoto-electrons released by the biasing light from the photo-sensitivecoating which is produced on the Wall of the tube during thesensitization of the photo-electric storage mosaic in the case of tubesof the iconoscope type, or the photo-cathode in the case of tubes of theimage iconoscope type. This collection of photo-electrons by the storagesurface tends to equalize the non-uniformity of the redistribution ofthe secondary electrons over the storage surface, and also tends toreduce the effect known as flare, by tending to neutralize the positivecharge resulting from the scanning on the bottom zone of the storagesurface. The efliciency of the improvement obtained in this way is verylow because only a small part of the photo-electrons released by thebiasing light can reach the storage surface, owing to their low initialvelocity.

The present invention has for its object to increase the efliciency ofthe improvement obtained by irradiating the tube with a bias light, andconsists in the provision of means for accelerating towards the storagesurface the photoelectrons released from the irradiated photosensitiveWall coating or surface, whereby a greater proportion of the releasedphoto-electrons can reach the storage surface. However, the accelerationshould be such that the photoelectrons will reach the storage surfacewith a relatively low velocity so that they are able to charge thatsurface negatively.

The invention also consists in a television 2 pick-up tube of the kindemploying high velocity scanning, in which a photo-sensitive surface isprovided adjacent the storage surface, together with means forilluminating the photosensitive surface, and wherein the collectorelectrode for collecting the secondary electrons released from thestorage surface is interposed between the latter and the photo-sensitivesurface and is arranged to be maintained at a positive potentialrelatively to the photo-sensitive surface.

According to a feature of the invention, the collector electrode isperforated, so that the photo-electrons released from thephoto-sensitive surface reach the storage surface through theperforations in the collector electrode and are accelerated towards thestorage surface by reason of the collector electrode being maintained ata more positive potential than the photo-sensitive layer.

In order that the invention may be more clearly understood, anembodiment thereof will be described by way of example with reference tothe accompanying drawing, which shows diagrammatically a pick-up tube ofthe image iconoscope type modified according to the present invention.

Referring to the drawing, the pick-up tube comprises an evacuatedenvelope I containing a semi-transparent photo-cathode 2 adjacent oneend of the tube and a storage target spaced therefrom and arrangedparallel to the photocathode 2. The target may comprise a mica sheet 3,of which the surface facing the photocathode 2 constitutes the storagesurface of the target. That surface may be coated with a material tomake it highly secondary emissive. The sheet 3 is backed with acontinuous conductive layer 4 forming the signal plate which isconnected externally of the envelope l to a load resistor 5, acrosswhich the signal potentials are developed and fed into a signalpreamplifier. Photo-electrons released from the photo-cathode 2 underthe influence of an optical image projected thereon are acceleratedtowards the storage surface 3 by the Wall coating 1 which is maintainedat a positive potential by connection to a source of potential such asbattery l3, and are focussed upon the storage surface by the field of animage focussing coil 8 surrounding the envelope 1. The storage surface 3is scanned in a line raster by a high velocity beam generated by anelectron gun 6 in an inclined side tube l' of the main envelope l', theside tube I being surrounded by the necessary coils l4 and I5 forfocussing and deflecting the beam. A collector electrode 9 is arrangedin the envelope l for collecting secondary electrons emitted from thestorage surface 3 both by the incident primary photo-electrons from thephoto-cathode 2 and by the incident electrons from the scanning beam.The construction and mode of operation of the tube as so far describedis conventional and well known in the art.

However, whereas in the conventional construction the collectorelectrode 9 is usually in the form of an internal conducting coating onthe wall of the envelope l, according to this invention the collectorelectrode 9 is constructed of a coarse mesh conductor which is ofcylindrical shape and is mounted surrounding the storage surface 3 andspaced a small distance from the internal surface of the envelope lwhich is coated with a photo-sensitive layer Ill.

The latter is irradiated by a light source ll, shown provided outsidethe envelope I. The source ll may be of annular form surrounding theenvelope I and layer IE3, or an annular array or group of light sourcesmay be similarly arranged. Precaution must be taken to avoid light fromthe source H from falling onto the photo-cathode 2, in order to avoidany additional noise component. Accordingly, the source H is preferablyprovided with suitable means such as a shield or light guides [2 or thelike in order to confine the illumination to the photo-sensitive layerIO.

A potential difference is maintained between the mesh 8 and thephoto-sensitive layer It, so that the mesh is positive with respect tothe photo-sensitive layer and accelerated photo-electrons emitted fromthe irradiated layer In. The value of this potential difference (whichmay, for example, be 5-8 volts) is within the range for which thesecondary emission ratio of the storage surface 3 will be less thanunity; that is to say, photo-electrons from the irradiated layer impingeupon the storage surface 3 with such velocities that the ratio of thenumber of secondary electrons released from the storage surface to thenumber of arriving photo-electrons is less than unity, whereby thestorage surface is negatively charged.

The mesh aperture of the electrode 9 and the distance between it and thephoto-sensitive layer 10 are made of suitable values to produce a smallfield penetration factor, so that the equilibrium potential of thestorage surface 3 resulting from the scanning is mainly determined bythe mesh potential. On the other hand, sufficient photo-electrons mustbe capable of penetrating through the mesh 9 and landing on the storagesurface in order to produce the wanted negative charge.

The. photo-emission from the photo-sensitive layer It] may be madenon-uniform, for example, by illuminating the layer i0 non-uniformlyalong its length as shown by the position of source H or along itscircumference, in order to obtain the desired charging of the storagesurface. For example, the illumination may be more intense at the zoneof the layer H] which will irradiate the bottom zone of the storagesurface 3 with photo-electrons, in order to compensate for flare.

If desired, and especially when using intermittent film projection intelecine transmission, the photo-sensitive layer in ,may be onlyintermittently illuminated, for example, .during only the fiybackperiods of the scanning beam, preferably the frame flyback periods, sothat the storage surface will be charged negatively only during theseflyback periods. This intermittent illumination may be effected byobturating the light source H by rotating or vibrating shutters orsimilar devices. For example, a shutter l6 may be rotated by motor I!and drive shaft l8 alternatively, as the light source, one or morecathode ray tubes or gas discharge tubes may be employed, the lightoutput of which can be pulsed and controlled by feeding thereto suitablevoltage or current pulses to operate the tube or tubes during theflyback periods.

Whilst the particular embodiment described applies the invention to atube of the image iconoscope type, the invention may be appliedsimilarly to a tube of the iconoscope type, the photoelectrons from thelayer [U in that case irradiating the photo-sensitive mosaic target ofthe iconoscope instead of the secondary emitting target of the imageiconoscope.

I claim:

1. In a television pick-up tube of the high velocity scanned type, thecombination :with a charge storage surface capable of storing energy inthe form of a charge pattern corresponding to the light distribution inan optical image, of a source of photo-electrons in the vicinity of saidsurface adapted to release photo-electrons in that vicinity, means forilluminating said source to cause it to release photo-electrons, acollector electrode in the vicinity of saidstorage surface forcollecting electrons released from said storage surface, said collectorelectrode being disposed between said source and said storage surface,and means for maintaining said collector electrode at a positivepotential relatively to said source for accelerating photoelectronsreleased from said .source towards said storage surface.

2. In a television pick-up tube of the high velocity scanned type, thecombination with a charge storage surface capable of storing energy inthe form of a charge pattern corresponding to the light distribution inan optical image,.of .a source of photo-electrons in the vicinity ofsaid surface adapted to release photo-electrons in that vicinity, meansfor illuminating said source to cause it to release photo-electrons, acollector electrode in the vicinity of said storage surface forcollecting electrons released from said storage surface, said, collectorelectrode being interposed between said source and said storage sur--face and being of perforate construction to allow photo-electronsreleased from said source to pass through said electrode, and means formaintaining said collector electrode at a positive potential relativelyto said source for accelerating photoelectrons released from said sourcetowards said storage surface. 7

3. A television pick-up tube comprising an evacuated envelope havingwithin it a photocathode upon which a light image is adapted to bedirected to release photo-electrons from said photo-cathode, a targetspaced from said photo cathode and comprising a secondary electronemitting storage surface facing said photo-cathode and forming elementalcondensers with a signal plate backing said storage surface, means foraccelerating and focussing photo-electrons released from saidphoto-cathode to impact upon said storage surface whereby to ejectsecondary electrons from said surface to form a charge pattern thereoncorresponding to the light distribution in the light image, means forscanning said surface with a high velocity electron beam to dischargesaid surface to equilibrium potential, a collector electrode in thevicinity of said surface for collecting secondary electrons emitted fromsaid surface, a photo-electric surface in the vicinity of said storagesurface for releasing photo-electrons towards said storage surface,means for illuminating said photo-electric surface to cause it to emitphoto-electrons, and means for accelerating photo-electrons releasedfrom said photo-electric surface on to said storage surface whereby toeject secondary electrons from said surface in such ratio to theincident photo-electrons as to charge said surface negatively.

4. A television pick-up tube comprising an evacuated envelope havingwithin it a photocathode upon which a light image is adapted to bedirected to release photo-electrons from said photo-cathode, a targetspaced from said photocathode and comprising a secondary emittingstorage surface facing said photo-cathode and. forming elementalcondensers with a signal plate backing said storage surface, means foraccelerating and focussing photo-electrons released from saidphoto-cathode to impact upon said storage surface whereby to ejectsecondary electrons therefrom to form a charge pattern thereoncorresponding to the light distribution in the light image, means forscanning said surface with a high velocity electron beam to dischargesaid surface to equilibrium potential, a collector electrode in thevicinity of said surface for collecting secondary electrons emitted fromsaid surface, a photo-electric surface in the vicinity of said storagesurface for releasing photo-electrons towards said storage surface,means for illuminating said photo-electric surface to cause it to emitphoto-electrons, said collector electrode being spaced close to saidphoto-electric surface and disposed in the path of photo-electronsreleased from said photo-electric surface and having a mesh constructionto allow released photo-electrons to passtherethrough, and means formaintaining said collector electrode at a positive potential relativelyto said photo-electric surface for accelerating photo-electrons releasedfrom said photo-electric surface on to said storage surface whereby toeject secondary electrons from said surface in such ratio to theincident photo-electrons as to charge said surface negatively.

5. A television pick-up tube as defined in claim 4, and furthercomprising means for shielding said photo-cathode from light from saidilluminating means.

RICHARD THEILE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,324,53 Pierce July 20, 19432,360,663 E'ddy Oct. 17, 1944 2,368,884 Schade Feb. 6, 1945 2,378,857Mayle June 19, 1945 2,451,005 Weimer et a1 Oct. 12, 1948

